The present invention concerns the field of equipment for motor vehicles, more particularly that of thermal-electric hybrid vehicles (comprising a heat engine, usually an internal combustion engine, and at least one electric motor) and relates to an automatic transmission, or automatic gearbox, optimized for such vehicles.
The invention relates more specifically to transmissions for hybrid vehicles that can operate purely by electric drive, in other words for vehicles that can be moved solely by the action of an electric motor and in which the heat-engine and electric-motor drives pass through the same transmission before being applied to the driving wheels.
For the above-mentioned type of vehicle, the state of the art currently offers two main types of architecture.
A first type of architecture corresponds to so-called Type “P2” parallel hybrids, which operate with an electric motor placed between the heat engine and the gearbox. A special decoupling clutch is provided that allows the heat engine to be disconnected from the drive chain.
In the event of decoupling, operation is of the purely electric type, the electric motor supplying the torque to the gearbox, instead of the heat engine. The electric motor can also serve as a transmission brake in normal braking situations: it is then in generator mode and produces electricity to recharge the battery. The gearbox then operates in a conventional manner, adapting the gear ratios to the optimal operating speed of the electric motor. In the absence of decoupling, when the heat engine is connected, the electric motor can be enlisted as a motor or generator, to provide a power boost for acceleration or to recharge the battery.
This first architecture is currently the most widely used, at least in the European market.
It has the advantage of being able to use an existing gearbox to the input of which an electric motor is connected together with a decoupling clutch enabling the heat engine to be disconnected from the traction drive. This means that it is not necessary to develop a special hybrid transmission system. Moreover, when the battery is flat and the vehicle can no longer be propelled except by means of the heat engine, a large number of gear ratios are always available, allowing the heat engine to operate at its optimum speed.
However, this first known solution has its drawbacks: for high electric motor powers (more than 40 kW, chiefly for plug-in vehicles, in other words rechargeable from an external source), it becomes difficult to integrate this motor in the gearbox, thus resulting in an increase in the overall size of the drivetrain. The space constraints for placing the motor between the engine and the gearbox means that expensive motors with permanent magnets must be used.
Moreover, it has been determined that the more the power of the electrical system increases in relation to that of the heat engine, the less the need for gear ratios to achieve the same vehicle performance.
Lastly, if the battery is flat and there is no more energy available to start the electric motor, a special starting system must be provided on the heat engine (such as a hydraulic torque converter or a friction clutch). In other words, the resulting system occupies too much space, is too expensive and too complex for the performance provided.
Examples of transmission constructions suitable for this first type of architecture are in particular disclosed in documents US 2008/011529 and WO 2008/141876.
A second type of architecture is known by the name of in-series or series/parallel hybrid.
In this second solution, a generator driven by the heat engine produces electricity that is used to run an electric motor that propels the vehicle (series operation). In most of the current arrangements, some of the power of the heat engine is however transmitted mechanically to the wheels (series/parallel operation).
Such a system is perfectly optimized for hybrid operation, with any number of gear ratios enabling the electric motors and the engine to run at optimum speeds with a minimum of gears and clutches (because there is no need to create multiple mechanical gear ratios). Moreover, there is no need for a special starting system on the heat engine, since starting can always be achieved in series mode.
By contrast, one major drawback lies in the extra weight and cost caused by using two electric motors, while only one is really used for the propulsion of the vehicle. Thus, in the case of series operation, if a 60 kW electric motor is used to propel the vehicle, a 60 kW generator must also be provided. In conclusion, twice the desired propulsion power must be embarked (see for example EP 1 386 771 and US 2012/174708).
In an attempt to counter these drawbacks and overcome the limitations of existing solutions, transmissions and gearboxes specially adapted to hybrid vehicles have in the meantime been developed and offered on the market, which include an electric motor as standard.
This new generation of transmissions usually has the following characteristics:                two to five gears than can be used to propel the vehicle by means of the heat engine;        one or two gears to propel the vehicle by means of the electric motor;        a method of vehicle takeoff or start-up by the heat engine, using the electric motor in generator mode like a variable speed drive (variable ratio operation mode, see for example DE 102010031026A1).        
The number of ratios associated with the electric motor in these new transmissions is adjusted to suit the needs of a hybrid vehicle (no superfluous equipment to achieve six or eight speeds). The possible provision of two electric speeds allows the operation of the electric motor to be adjusted within its ranges of speed and torque where it is the most effective, thus enabling the use of asynchronous motors that are less expensive than permanent-magnet motors, but with a smaller optimum efficiency zone. The variable ratio start-up method economizes on a vehicle takeoff device on the engine (converter or clutch with a high energy-dissipation capacity), while still offering a possibility of takeoff other than electric, particularly when the batteries have reached their minimal charge threshold (for example after repeated start-ups in congested traffic).
Nevertheless, these new developments in transmission specifically designed for hybrid vehicles also involve complex constructions, in particular requiring at least three planetary gear sets and at least four selective torque transmission devices (clutches or coupling mechanisms).
Moreover, these new developments do not allow a simple transition, in terms of construction, from a basic version, with more limited possibilities, to a more sophisticated version, with increased possibilities in terms of number of available gear ratios.